1) Field of the Invention
The invention relates to crosslinkable polymer powder compositions, to processes for their preparation and to their use.
2) Background Art
In practice, it is often advantageous if the user is provided with ready-to-use systems or mixtures, for example, as adhesives or coating compositions. This applies, for example, to water-soluble or water-dispersible solid adhesives or coating compositions which crosslink in the presence of strong acids. If crosslinking is effected by splitting off water, such systems comprising crosslinkable binder and acid in the anhydrous state are particularly susceptible to premature crosslinking.
Although there are a number of potent compounds in solid form which are not or are only very weakly hygroscopic and give a strongly acid reaction in the presence of water such as, for example, AlCl.sub.3 .times.6H.sub.2 O, NaAlCl.sub.4 and Cr(NO.sub.3).sub.2 .times.9H.sub.2 O, all these have one thing in common: traces of water are sufficient to liberate acid which then triggers the crosslinking reaction. The shelf life of such mixtures is therefore very limited since, in the case of the products in question, complete exclusion of water (or other proton-containing solvents such as alcohols) is possible only with difficulty and in many cases, is not possible at all.
Examples of aqueous dispersions of copolymers which can be crosslinked in the presence of acids or metal salts are mentioned in DE-A 2620738 (U.S. Pat. No. 4,118,357) and DE-A 3942628 (U.S. Pat. No. 5,296,532). These are aqueous dispersions of vinyl ester copolymers with N-methylol functional groups which comprises H.sub.2 O soluble metal salts or acids such as HCl or HNO.sub.3, as the hardener additive. Water-redispersible powders of plastics which are accessible by drying such dispersions can be obtained in anhydrous form only with difficulty. If drying of aqueous polymer dispersions is too intense, the redispersibility of the polymer powders is adversely influenced. Furthermore, the dispersion powders have a protective colloid content; the usual protective colloids such as polyvinyl alcohol, starch, starch derivatives or cellulose derivatives containing reactive protons which liberate acid from the hardener salts mentioned which, in the end, again leads to premature crosslinking of copolymers.
There was therefore the object of providing water-dispersible or water-soluble polymer powder compositions which can be crosslinked or condensed by acid, do not tend to undergo premature crosslinking or condensation even with a residual moisture content contained in the powder composition, and only crosslink when the powder composition is dispersed or dissolved in water.